Biosensing instruments that employ disposable sample strips enjoy wide consumer acceptance. Such instruments are employed for the detection of various analyte levels in blood samples, e.g., glucose and cholesterol. In general, such instruments provide accurate readings if the user is careful to follow the instrument's directions. Often however, the user is careless in the use of either the sample strip or the instrument and an erroneous reading results. Accordingly, significant efforts have been taken by instrument manufacturers to reduce the potential for errors during the use of such instruments.
Even if a biosensing instrument and sample strips are employed properly, the presence of a manufacturing defect in either will cause erroneous readings. Thus, while great care is taken in the production of such instruments and sample strips, there is a need to incorporate analytical procedures in the instrument that enable instrument malfunctions, sample strip irregularities and user errors to be detected so as to prevent erroneous analyte readings.
The prior art includes a number of disclosures of biosensing instruments that employ disposable sample strips. In U.S. Pat. No. 5,108,564 to Szuminsky et al., a biosensing instrument is disclosed that measures glucose concentrations in blood. The instrument depends upon a reaction wherein glucose, in the presence of an enzyme, catalyzes a reaction of potassium ferricyanide to potassium ferrocyanide. After the reaction has completed, a voltage is applied across a reaction zone and causes a reversal of the reaction with an accompanying generation of a small, but measurable current. That current is termed the Cottrell current and, in dependence upon the concentration of glucose in the reaction zone, follows a predetermined curve during the reverse reaction. A reading of the Cottrell current is converted into an indication of glucose concentration. The instrument also senses an impedance across the reaction zone and determines when a blood sample has been emplaced therein by detecting a sudden change in current flow. At such time, an incubation period is commenced, followed by application of a potential across the reaction zone and measurement of the Cottrell current.
European Patent Application 0 471 986 A2 of Tsutsumi et al. discloses a blood glucose measurement system that employs disposable sample strips. The Tsutsumi et al. system detects the presence of a blood sample by sensing a resistance across a pair of electrodes. It further employs a plurality of sample-like strips, each having a specific resistance value which distinguishes it from other strips. Each of those strips has a particular application, i.e., for use during an adjustment mode of the instrument, during an error compensation mode, during a calibration mode, etc.
U.S. Pat. No. 4,999,582 to Parks et al., assigned to the same Assignee as this application, describes a biosensor electrode excitation circuit for determining if a sample strip has been properly inserted into a meter and if at least one electrode on the sample strip exhibits a proper level of contact resistance.
U.S. patent application Ser. No. 07/451,309, filed Dec. 15, 1989 to White, entitled "Biosensing Instrument and Method" and assigned to the same assignee as this application, teaches a biosensing instrument which employs the "Cottrell" curve relationship to determine glucose concentrations. In that instrument, current flow is proportional to the concentration of an analyte in the test cell; however, when something is amiss in the test cell, the current that results may bear no relationship whatever to analyte concentration. White found that a relationship exists that enables a determination to be made whether current flow through a reaction zone is, in fact, following the Cottrell relationship. More specifically, the ratio of the square roots of succeeding sample times, for all analyte concentration curves, has been found to inversely approximate the ratio of the measured Cottrell currents at those same sample times. If over succeeding time periods, the ratios are equal (within limits), the measurement system is properly following the Cottrell relationship. If the ratios found are not equal, the measurement is disregarded.
U.S. Pat. No. 4,940,945 to Littlejohn et al. describes an interface circuit for use in a biochemical sensing instrument. A disposable cartridge is employed that includes a pair of electrodes across which resistance measurements are taken. Circuitry is disclosed for sensing the presence of a fluid sample by an initial resistance measurement, and also the level of fluid in the cartridge.
U.S. Pat. No. 4,420,564 to Tsuji et al. describes a blood sugar analyzer that employs a reaction cell having a fixed enzyme membrane sensor and a measuring electrode. The Tsuji et al. system includes several fail/safe procedures, one to determine that the reaction is taking place within specifically defined temperature limits and a second to determine that the reaction current remains within a predetermined range.
The above noted prior art indicates that biosensing meters have had the ability to determine when a biological sample is placed in a reaction zone. However, the prior art has not addressed the problem of the presence of an insufficient amount of the sample to fully wet enzymatic reactants present in a reaction zone. Furthermore, while a test is available to determine that a reaction is following the Cottrell relationship (as described in the aforesaid patent application to White), additional, confirming, tests are desirable to assure that the reaction is in fact, following the Cottrell relationship.
Accordingly, it is an object of this invention to provide a biosensing meter with means for performing a plurality of fail/safe tests during the course of an analysis of a biological sample.
It is another object of this invention to provide a biosensing meter with means for determining that adequate amount of a sample has been placed in a sample strip's reaction zone.
It is a further object of this invention to provide means for determining that a biological sample is reacting in accordance with the Cottrell relationship during the course of the reaction and, if not, causing resulting readings to be ignored.